1. Field of the Invention
The present invention relates to a method of evaluating a contact area ratio of an electrode tip when carrying out resistance welding using the electrode tips, a method of evaluating an internal resistance of workpieces, a method of evaluating an attenuation rate of ultrasonic waves, and a method of determining an inclined condition of the electrode tip. The present invention also relates to a method and apparatus for detecting the position of an interface of a molten portion, which is generated when workpieces are placed mutually in abutment and spot welding is carried out thereon.
2. Description of the Related Art
When spot welding, which forms one type of welding technique, is performed, as is well known, workpieces that have been placed mutually in abutment with each other are gripped between a pair of electrode tips, and by causing a current to flow between the electrode tips, welding in the form of spot welds on the workpieces is carried out.
Spot welding typically is performed, for example, by a welding gun, which is arranged at an end of a robot arm of a teachable robot. More specifically, the robot, which has been taught beforehand, is operated (moved) initially so as to enable workpieces to be inserted between electrode tips that are disposed on an openable/closable clamp of the welding gun. Next, by closing the clamp, the workpieces are gripped between the electrode tips. In this state, the electrode tips are energized and current is caused to flow therebetween, whereby a molten portion is formed internally within the workpieces. Lastly, a spot weld is formed on the workpieces accompanying solidification of the molten portion.
In spot welding implemented in the foregoing manner, the growth and solidification of the molten portion, i.e., what timing the molten portion grows and solidifies at, is inspected. As one such detection method, the technique disclosed in Japanese Patent Publication No. 59-014189 is known. More specifically, in this technique, an ultrasonic oscillator for transmitting and receiving ultrasonic waves is incorporated into one of the electrode tips. Ultrasonic waves are transmitted from the ultrasonic oscillator, whereupon the ultrasonic waves are reflected by a reflection-inducing surface disposed on the other remaining electrode tip, and the reflected ultrasonic waves are received by the aforementioned ultrasonic oscillator.
When resistance welding is performed repeatedly with respect to a plurality of workpieces, the electrode tips gradually suffer from wear and abrasion. As a result, the contact areas between the electrode tips and the workpiece undergo change, and along therewith, the area on the electrode tip that is capable of transmitting ultrasonic waves also changes. In the case that such a condition occurs, with the technique of Japanese Patent Publication No. 59-014189, which is premised on the contact area between the electrode tips and the workpiece remaining constant, the detection accuracy of the interface position of the molten portion, and consequently the accuracy in calculating the speed at which the molten portion develops and grows, are deteriorated.
Further, if the technique of Japanese Patent Publication No. 59-014189 is implemented to evaluate what timing the molten portion is growing at, when the electrode tips abut with respect to the workpiece in an inclined condition due to inappropriate teaching of the robot, disagreement between the growth speed estimated when the robot was appropriately taught, and the measured growth speed occurs. In such a case, since the person conducting the evaluation is riot informed that the electrode tips are in abutment with respect to the workpiece in an inclined condition, there is a problem in that it cannot be evaluated whether the growth speed of the molten portion is appropriate or not.
On the other hand, a method and apparatus for detecting the position of an interface of a molten portion, for investigating what timing the molten portion has grown and solidified at, has been proposed in Japanese Laid-Open Patent Publication No. 2007-248457. More specifically, with the detection method and apparatus of Japanese Laid-Open Patent Publication No. 2007-248457, ultrasonic waves are directed toward the molten portion from a transmitter-receiver incorporated into a welding tip, whereby reflected waves that are reflected from the molten portion, or transmissive waves that have passed through the molten portion, are detected.
In the conventional technique of Japanese Laid-Open Patent Publication No. 2007-248457, the molten portion is detected based on a hypothesis that the velocity of ultrasonic waves is constant. However, according to keen and diligent investigations carried out by the present inventors, it has been found that the velocity of such ultrasonic waves actually becomes delayed (i.e., slower) accompanying a rise in the temperature of the workpiece. Accordingly, as the temperature of the workpiece changes following advancement of the spot welding, there is a concern that the detection accuracy in detecting the position of the molten portion may become deteriorated, if only somewhat, using such a conventional technique.